[hpsdr] GPS 1PPS board

Mon Apr 6 15:38:59 PDT 2009

Not really -- no matter what system you use, the short term stability 
(less than 1000 seconds or so) will be dominated by the XO performance. 
  The GPS can't improve that.  What the GPS does is (slowly) steer to 
the right frequency there, and hold it there over long time periods. 
The magic is setting things up so that the GPS takes over at the point 
where its performance matches that of the XO.

It doesn't take any magic circuitry to do that -- G3RUH gets truly 
amazing performance from a very simple analog loop circuit, but he uses 
a fairly expensive OCXO.

Most of the complexity in the fancy GPSDOs is to get what's called 
holdover performance -- basically, to learn the XO's aging rate and even 
temperature response so that if the GPS signal is lost the system can 
keep steering the frequency to hold the performance for a day or more. 
  That holdover performance isn't as big a deal now because there are 
plenty of GPS satellites up there, so a design like the G3RUH can just 
ignore it.

John
----

Henry Vredegoor said the following on 04/06/2009 06:28 PM:
> Hi John, All,
> 
> So if I understand you correctly, you trade a more expensive, higher quality
> (short term stability) XO and simple cheap GPS/locking circuitry for a
> (maybe-) less expensive, lesser quality XO and more complex, better
> GPS/locking circuitry?
> 
> I am speaking of frequency control as primary use for HPSDR and precision
> timing as a nice bonus for secondary use.
> I can however imagine that ADC sample time stamping for sophisticated
> receiving techniques could require more precise and higher resolution
> timing?
> 
> 73,
> 
> Henry.
> 
> 
> 
> 
> 
> 
> 
>> -----Original Message-----
>> From: hpsdr-bounces at lists.hpsdr.org 
>> [mailto:hpsdr-bounces at lists.hpsdr.org] On Behalf Of John 
>> Ackermann N8UR
>> Sent: maandag 6 april 2009 23:02
>> To: Chris Salinas
>> Cc: SDR
>> Subject: Re: [hpsdr] GPS 1PPS board
>>
>>
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> If I did the math correctly, that value is 1x10e-10.  That's 
>> actually a 
>> couple of orders of magnitude less than the accuracy of the GPS 
>> constellation; it's generally accepted that averaging over a 
>> day or so, 
>> you can get to within parts in 10e-13.
>>
>> 1x10e-10 is a fairly common performance spec for simple 
>> GPSDOs.  Better 
>> ones, like the surplus HP and Trimble units many hams have, are more 
>> like 1x10e-12.  But both are way better than any HF or VHF work could 
>> ever require, so most of this discussion is in "time-nuts" 
>> territory. :-)
>>
>> But the key thing is that any frequency system based on GPS has to 
>> average over a long time period, usually 1000 seconds or 
>> more, because 
>> the jitter on the GPS signal is much worse than 1x10e-10.  
>> Even really 
>> good GPS timing receivers like the Motorola M12+ have about 30 
>> nanoseconds of jitter.  A unit that has 1 microsecond of 
>> jitter bounces 
>> around by 1x10e-6, or 1 Hz at 1 Mhz, potentially every second.
>>
>> So, you need to average the signal using a PLL with a very long time 
>> constant, and use that to steer a crystal oscillator that is 
>> stable over 
>> shorter time periods, to make a workable frequency standard 
>> out of GPS.
>>
>> And there the key is that a 10 kHz pulse rate makes the PLL easier to 
>> implement -- but in the end it won't be any more accurate 
>> than the 1 PPS 
>> signal because the 10 kHz pulses are themselves only 
>> "disciplined" once 
>> per second by the GPS engine.
>>
>> 73,
>> John
>> ----
>>
>> Chris Salinas wrote:
>>> John,
>>> I just read a website on this TU00 model and that they measured the 
>>> accuracy of +/- .000001 Hz at the 10 Khz pin. Too me it 
>> sounds stable 
>>> enough for a majority of work.
>>>  
>>> I ordered one yesterday just too play with and do some 
>> measurements. Can 
>>> always use it for APRS if it doesn't hold water.
>>>  
>>> I'll let you all know what I find.
>>>  
>>> Chris N0TTW
>>>
>>> --- On *Mon, 4/6/09, John Ackermann N8UR /<jra at febo.com>/* wrote:
>>>
>>>
>>>     From: John Ackermann N8UR <jra at febo.com>
>>>     Subject: Re: [hpsdr] GPS 1PPS board
>>>     To: "Henry Vredegoor" <henry.vredegoor at gmail.com>
>>>     Cc: "'Graham / KE9H'" <KE9H at austin.rr.com>, "'Chris Salinas'"
>>>     <n0ttw at yahoo.com>, "'HPSDR discussion list'" 
>> <hpsdr at lists.hpsdr.org>
>>>     Date: Monday, April 6, 2009, 3:09 PM
>>>
>>>     I'd put it a little differently:  the 10kHz signal does 
>> not make it
>>>     possible to get *better* performance than a GPS with a 
>> 1 PPS signal,
>>>     but it allows much simpler circuitry since filtering a 
>> 10 kHz signal
>>>     is much easier than filtering 1 PPS, particularly if 
>> using analog
>>>     components.
>>>
>>>     However it's important to realize that the GPS part of any GPSDO
>>>     affects only the long term frequency stability.  When 
>> measuring over
>>>     time periods shorter than around 1000 seconds, the 
>> performance of
>>>     the XO is paramount.  One of the problems with building a cheap
>>>     GPSDO is that good XOs are expensive, and therefore the 
>> short term
>>>     stability is pretty much  tied to the builder's budget 
>> or scrounging
>>>     skills.
>>>
>>>     The control loop should (a) optimize the "cross-over" 
>> point where
>>>     the XO stability matches the GPS system stability; and (b) avoid
>>>     messing up the short term stability through excess 
>> noise applied to
>>>     the XO's tuning voltage.
>>>
>>>     John
>>>     ----
>>>
>>>
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